The use of hollow monofilaments in connection with reverse osmosis and other separation and purification procedures is old and well known as shown in U.S. Pat. No. 3,422,008, issued Jan. 14, 1969, to Dow Chemical Company, and as shown in report PB233,102 of the Monsanto Research Corporation, dated September, 1973, to the U.S. Department of the Interior.
Hollow filament separatory modules generally consist of a large number of hollow filaments with permeable walls arranged in a cylindrical bundle inside a pressure vessel and sealed in a potted barrier or tube sheet at one or both ends of the bundle. Ends of the filaments are exposed and sliced open at their ends outside the tube sheet to provide fluid passages from the inside of the filaments.
A variety of methods is known to prepare the filament bundle assemblage. Many of these are intricate and many involve complex steps of winding with special machinery and utilization of porous sheets which act as support members.
In a module, a properly assembled multifilament array must meet a number of specifications. Among these is a geometric arrangement which provides a fairly high packing density and which at the same time minimizes channeling of the feed fluid as it passes among the filaments. Therefore, some method of keeping filaments from packing densely in certain areas and loosely in others is desirable. In other words, packing density of the filament array should be both very uniform and in the order of about 25% to 60%, if some of the main advantages of the use of hollow filaments are to be achieved. In view of these considerations, the use of various filament assembly techniques is frequently unattractive.
A bundle of filaments assembled as a collection of parallel axially aligned filaments loses some effectiveness because there is an opportunity for the occurrence of low flow resistance axial channels between filaments at some locations, with a resulting short circuiting of the fluid flow, and an inequality of flow distribution to and through the walls of all of the individual filaments. There is also the likelihood of neighboring filaments to be pressed against each other tangentially for long lengths, thus reducing the effective surface area of each such filament.
An advantageous geometry to attain the desired result has been described in U.S. Pat. No. 4,045,,851, as comprising an annular bundle of hollow filaments wound in a plurality of layers at a selected helix angle with alternate being of opposite helix direction. Further benefits of having helical intermeshing orientation of filaments to one another is shown in U.S. Pat. No. 4,105,548, where a three-dimensional network in spiral wound structure of hollow filaments in multiple layers is described. In both these methods, however, the helical structure is achieved by winding the filaments on a mandrel, resulting thereafter in the formation of an annular bundle of filaments.
While these methods provide the relatively uniform fine pore distribution of spaces among the filaments desired, in both cases the preparation of an annular bundle tends to offset some of the desired advantages of the use of a multi-filament membrane system. First, the presence of a core element filling the region of the annular bundle tends to reduce the effective total membrane surface area attainable within the operating module of which the filaments are a part. Second, the preparation of an annular bundle by wrapping fibers helically upon a core of mandrel element inherently limits the steepness of the helix to that attainable without undue slippage restrained only by resistance due to fiber on fiber. It would be desirable to provide helically wound fibers in bundles absent a central core and at extremely steep helix angles for some applications. This can be achieved to some limited extent by methods contemplated in the cited U.S. Pat. No. 4,045,851. There nevertheless remains some residue of a hollow central region by the method described in that patent and it is therefore of advantage to pursue alternate methods to attain the desired result.